1. Field of the Invention
The present invention relates to a display device using TFTs (Thin Film Transistors), such as an OEL (Organic Electroluminescence) display device.
2. Description of Related Art
A conventional active matrix display device using TFTs will be described with reference to FIG. 1, which shows an equivalent circuit for one pixel. As shown in FIG. 1, each pixel of a display device having an OEL as a display element basically comprises a first transistor Tr1 for switching, a second transistor Tr2 for driving an element, and a storage capacitor C.
The drain terminal (D) of the first transistor Tr1 is connected to an input line for a data voltage signal (Vdata) while the gate terminal (G) of the first transistor Tr1 receives an externally input gate signal (Gate Sig). Further, the source terminal (S) of the first transistor Tr1 is connected with one end of the storage capacitor C and with the gate terminal (G) of the second transistor Tr2. The other end of the storage capacitor C is connected with a VSC line.
A power source voltage PVdd is applied to the source terminal (S) of the second transistor Tr2, and the drain terminal (D) of the second transistor Tr2 is connected to an anode of the OEL element.
In conventional display devices, a data voltage signal corresponding to a desired gray scale value is applied to the drain terminal (D) of the first transistor Tr1 and a gate signal is input to the gate (G) of the transistor Tr1, so that the first transistor Tr1 is turned on and a charge in accordance with a voltage value of the data voltage signal is held in the storage capacitor C. The conducting state (resistance) between the source (S) and the drain (D) of the second transistor Tr2 is controlled by the amount of charge held in the storage capacitor C. Further, the OEL element is driven by the current value which is determined by the power source voltage PVdd and the controlled resistance. More specifically, the resistance value of the second transistor Tr2, and thus the current value applied to the OEL, is controlled by the data voltage signal input to the first transistor Tr1, so that the OEL emits light with a brightness that will produce a desired gray scale image.
Active matrix display devices as described above have attracted attention as having promise for the next generation of displays because they are of a self-emission type and thin, and, because they can be driven with less power, they reduce power consumption. However, these display devices are still in the stage of research and development, and no devices have yet been proposed for inspecting defects of each pixel precisely and effectively with a low cost.
Active matrix TFT LCDs, display devices having a TFT for each pixel, are widely used today. In a TFT LCD, the liquid crystal is controlled by a voltage applied thereto via each pixel TFT. Accordingly, when inspecting the TFT substrates in such LCDs, commonly the charge accumulation state on each storage capacitor C is measured to determine the quality of each transistor.
However, when controlling the emission gray scale of OELs by means of conventional current value control as described above, there has been problem in that, while the brightness of an emissive element is controlled by a source-drain current of the second transistor Tr2 whose gate voltage is controlled by a voltage held by the first transistor Tr1 and the storage capacitor C, a specialized measuring device is required in order to measure this current value for inspection purposes. In other words, conventional testers cannot be used for inspection, making it difficult and expensive to search for defects by perform inspections corresponding to actual display states.
The present invention was conceived in view of the aforementioned problems of the related art and provides a display device simplifying performance of defect inspection corresponding to the actual display state.
In order to overcome the above-described problems, in accordance with the present invention, there is provided an active matrix display device in which each of pixels comprises a display element; a first transistor for switching; a storage capacitor for holding a voltage signal supplied thereto via said first transistor, when the first transistor is ON; a second transistor for driving an element, which supplies power from a power source line to the display element, in accordance with the voltage signal which is held by the storage capacitor and is applied to the gate of the second transistor; and an additional capacitor which is connected such that a charge is accumulated therein by a current flowing from the second transistor to the display element.
In accordance with another aspect of the present invention, the additional capacitor is used for inspection of the active matrix display device and a matrix array substrate having the pixels formed on a substrate. Here, it is also preferable that the display element is an emissive element which emits light with brightness determined by the supplied power, and that the additional capacitor controls an amount of power supplied to the display element within a unit time period to thereby control emission brightness of the display element.
In order to overcome the foregoing problems, there is also provided a method of inspecting a display device as described above, the method comprising the steps of driving each of the pixels to accumulate a charge in the additional capacitor; measuring an amount of charge accumulated in the additional capacitor; and inspecting uniformity among currents supplied to each display element using the amount of charge measured for each display element.
In accordance with another aspect of the present invention, there is provided an active matrix semiconductor device in which each cell comprises a cell element; a first transistor for switching; a storage capacitor for holding a voltage signal supplied thereto via the first transistor, when the first transistor is ON; a second transistor for driving an element, which supplies power from a power source line to the cell element, in accordance with the voltage signal which is held by the storage capacitor and is applied to the gate of the second transistor; and an additional capacitor which is connected such that a charge is accumulated therein by a current flowing from the second transistor to the cell element.
According to the present invention, an active matrix device may be configured as follows. Namely, in each pixel or cell, when a first transistor for switching is ON, a storage capacitor holds a voltage signal supplied thereto via the first transistor. The voltage signal thus held in the storage capacitor is applied to the gate of a second transistor, which then supplies power from a power source line to a display element in accordance with the voltage signal. An additional transistor is further provided and connected such that it accumulates a charge by a current flowing from the second transistor to the display element. With this structure, by measuring the amount of charge accumulated in the additional capacitor after the display element is driven, it is possible to directly inspect the amount of current supplied to the display element via the second transistor, and defect inspection corresponding to the actual display state can be easily performed.